The invention relates to a plethysmographic measuring chamber for receiving the body of a laboratory animal for measuring various respiratory parameters, of the kind comprising a tubular body having a frusto-conical portion at its front end terminating in an aperture for admitting a flow of gas for inhaling, the chamber being bounded at the inlet opening by two axially separated annular seals having respective openings for receiving the animal's snout, whereas the other end of the chamber is bounded by a piston for adjusting the volume of the chamber to the size of the animals.
Chambers of this kind are used for measuring various respiratory parameters of a laboratory animal subjected to tests by inhalation of gas or aerosols formed from various substances. The gas flow resulting from variations in the volume of the animal's body is measured by a transducer and the resulting signal is processed in dependence on the parameters which it is desired to know. This method of measurement and treatment of the detected signal is described in detail in an article "Measurement of respiratory patterns in rodents using whole-body plethysmography and a pneumotachograph" published in Laboratory Animals No. 15 (1981), pages 137-140.
Clearly, the accuracy of measurement cannot be ensured unless the volume of gas in the plethysmographic chamber varies only with variations in volume of the animal's body resulting from the volume of inhaled gas or aerosol. If the gas or aerosol to be inhaled is made up of irritating substances, the animal instinctively tries to escape the flow of gas or aerosol sent to it through the inlet aperture in the tubular body, towards which its snout is directed. These efforts by the animal to pull back are limited by the piston but induce erratic variations in the volume of the plethysmographic chamber, resulting in considerable noise in the signal recorded by the transducer. The noise is difficult to filter and distorts the calculated parameters and consequently interferes with evaluation of the respiratory conditions of the animal subjected to the inhalation test.
The aim of the invention is at least partly to obviate these disadvantages so as to make the measurement more precise and therefore more reliable.